This invention relates to a power supply circuit for driving a motor, which can be connected to AC power sources providing different source voltages.
Commercial AC power sources available in various parts of the world provide different voltages. Therefore, domestic electric products such as electric shavers and cassette tape recorders sometimes cannot be powered by a commercial AC power source in a foreign country. Accordingly, there have been attempts to develop a power supply circuit for a motor to be used in an electric product, such as an electric shaver or a cassette tape recorder, which can be powered by different commercial AC power sources.
FIG. 1 shows a prior art power supply circuit for a motor driver of the type mentioned. It includes a full-wave rectifier circuit 2 for rectifying an AC voltage from an AC power source PS, capacitors C1 and C2 connected in series between the output terminals of the full-wave rectifier circuit 2, a switch SW1 connected between the connection point between the capacitors C1 and C2 and one of the input terminals of the full-wave rectifier circuit 2, and a half-bridge type DC-DC converter 4 for stepping down the DC voltage between the output terminals of the full-wave rectifier circuit 2. A chargeable battery e is connected between the output terminals of the converter 4, and also a motor 6 as a load is connected in series with a switch SW2 between these output terminals.
The half-bridge type DC-DC converter 4 has a well-known construction including npn transistors TR1 and TR2, primary and secondary windings L1 and L2 inductively coupled, feedback windings FL1 and FL2 inductively coupled with the primary winding L1, and diodes D1 and D2. The npn transistors TR1 and TR2 have their current paths connected in series between the output terminals of the full-wave rectifier circuit 2. The primary winding L1 has one terminal connected to the connection point between the npn transistors TR1 and TR2 and the other terminal connected to the connection point between the capacitors C1 and C2. The feedback windings FL1 and FL2 are respectively connected between the bases and emitters of the respective transistors TR1 and TR2. The diodes D1 and D2 have their anodes connected to the respective opposite ends of the secondary winding L2 and their cathodes connected together. The output voltage of the converter 4 appears between the connection point of the diodes D1 and D2 and a neutral point NP of the secondary winding L2.
With the motor power supply circuit shown in FIG. 1, a DC voltage V1 obtained from the full-wave rectifier circuit 2 appears between the output terminals of the series capacitor circuit of the capacitors C1 and C2 when the switch SW1 is in the OFF position. The DC-DC converter 4 generates a voltage equal to 1/n (n being the turn ratio between the primary and secondary windings L1 and L2) times the DC voltage V1. When the switch SW2 is in the OFF position, the battery E can be charged, while when it is in the ON position, the motor 6 can be driven.
Further, when the switch SW1 is in the ON position, the half-wave rectifier circuit 2 functions as a voltage doubler rectifier. In this case, a DC voltage V2 is supplied to the converter 4, and the converter 4 generates a DC voltage V2/n.
The switch SW1 may be set ON when the motor power supply circuit shown in FIG. 1 is connected to, for instance, a 100-V AC power source, while it may be set OFF when the power supply circuit is connected to a 200-V AC power source. By so doing, a substantially equal effective voltage is applied to the DC-DC converter 4 when the 100-V AC power source is connected to the power supply circuit and when the 200-V AC power source is connected. The battery E or motor 6 can thus be supplied with a predetermined voltage. As has been shown, the motor power supply circuit can be connected to two different AC power sources. However, it is necessary that the ratio of the effective output voltages in these two cases be 2:1. That is, it cannot be adapted for use with 100-V and 150-V AC voltage, for instance. In addition, the switch SW1 has to be operated according to the AC power source that is used with the power supply circuit. If the switch SW1 is operated erroneously, the motor 6 may fail to be driven or may be damaged.